1. Field of the Invention
The present invention relates to an optical fiber cable in which an optical fiber tape unit or a stack of a plurality of optical fiber tape units is accommodated in a spiral groove the direction of which is inverted periodically.
2. Description of the Related Art
An optical fiber cable having a spiral groove the direction of which is inverted periodically is known, for example, by Unexamined Japanese Patent Publication (kokai) No. Sho-63-301911.
FIGS. 5A and 5B are explanatory diagrams for explaining a conventional optical cable designed to have a spiral groove the direction of which is inverted periodically; FIG. 5A is an explanatory diagram taking a view from section, and FIG. 5B is an explanatory diagram taking a view from side. This explanatory diagram is based on the disclosure of the above-mentioned Publication, showing a so-called SZ-twisted optical fiber cable, that is, an optical fiber cable having a reverse-lay groove. In the drawing, the reference numerals 41 and 42 represent a center member and a groove, respectively. The groove 42 is cut spirally in the outer circumference surface of the center member 41, for example, a spacer, having an approximately circular section, and a not-shown coated optical fiber is accommodated in this groove 42. The spiral direction of the groove 42 is inverted periodically at a period P in the longitudinal direction of the center member 41. In FIG. 5B, there are two inverted positions, F1 and F2, and the difference between the rotation angles of the inverted positions F1 and F2 is an inversion angle .phi. in the circumferential rotation angle .theta. of the center member. Incidentally, FIG. 5D is a perspective view showing one example of the SZ-twisted optical fiber cable.
FIG. 6 is a sectional view of a conventional optical cable having spiral grooves the direction of which is inverted periodically. In the drawing, the reference numeral 51 represents a spacer; 52, a center tensile-resistance body; 53, grooves; 54, optical fiber units; 55, a pressing winding; and 56, a sheath. This drawing is based on the disclosure of the above-mentioned Publication. The four grooves 53 are cut into the spacer 51 which is a center member, and the optical fiber unit 54 is accommodated in each of the grooves 53. Further, the pressing winding 55 is given to the optical fiber units 54, and the outer circumference of the pressing winding 55 is coated with the sheath. In the above-mentioned Publication, only an embodiment in which three bare optical fibers are used in each optical fiber unit 54 is disclosed as a specific example. In this conventional cable, if an optical fiber tape unit or a stack of a plurality of optical fiber tape units is used as the optical fiber unit 54, bending distortion is produced in optical fibers in the optical fiber tape to thereby bring a problem in reliability of the optical fibers.
FIGS. 7A and 7B are explanatory diagrams for explaining the state where an optical fiber tape is accommodated in a conventional slot-type optical fiber cable; FIG. 7A shows the sectional shape in a grooved portion, and FIG. 7B shows the sectional shape of an optical fiber tape unit. In the drawing, the reference numeral 61 represents a spacer; 62, a groove; 63, a stack of a plurality of optical fiber units; 63a, an optical fiber tape unit; 64, a left wall; 65, a right wall; 66, a bottom portion; 67, an optical fiber; 68, a primary coating; and 69, a tape coating. This slot-type optical fiber cable is not an SZ-twisted optical fiber cable, but a S-twisted or Z-twisted optical fiber cable grooved in a straight line or having a not-inverted direction of twist.
In FIG. 7A, one or a plurality of grooves 62 are cut into the outer circumference of the spacer 61 having an approximately circular section. The stack of optical fiber tape units 63 is accommodated in this groove 62. Although a pressing winding or a sheath is often given thereto, they are omitted in the illustration. The section of this groove 62 is rectangular, and each of the left wall 64, the right wall 65 and the bottom portion 66 is straight. The side walls are perpendicular to the bottom portion, and the depth of the groove 62 is constant. The width between the left wall 64 and the right wall 65 may be slightly extended in the direction of the opening portion of the groove 62. In FIG. 7B, in the optical fiber tape unit 63a, the primary coating 68 is given to each of the optical fibers 67, and the four cores of the optical fibers are gathered and then coated with the tape coating 69. The stack of optical fiber tape units 63 shown in FIG. 7A is constituted by, for example, five layers of such optical fiber tape units 63a. The plane portion of the lowermost layer of them is put on the groove bottom portion 66, and the plane portions of the second layer et seq. are put one on one successively.
FIGS. 8A to 8C ar explanatory diagrams for explaining bending distortion of an optical fiber tape; FIG. 8A is an explanatory diagram for explaining the case where a desired linear body is bent, FIG. 8B is an explanatory diagram for explaining the case where the optical fiber tape is bent within a plane perpendicular to the tape plane, and FIG. 8C is an explanatory diagram for explaining the case where the optical fiber tape is bent within the tape plane. In the drawings, parts the same as those in FIGS. 7A and 7B are referenced correspondingly, and their description will be omitted. The reference numeral 71 represents a desired linear body, and 72 a center line of bending.
As shown in FIG. 8A, in the case where the desired linear body 71 is bent, let the radius of curvature in the neutral line of bending 72 R, distortion by bending .epsilon. and length from the neutral line of bending L at a special point P of the optical fiber core 71, and then .epsilon. is L/R. Accordingly, as shown in FIG. 8B, in the case where the optical fiber tape 63a is bent within a plane perpendicular to the tape plane where the optical fibers 67 (see FIG. 7C) are arrayed, the length from the center line of bending 72 is H/2 in the maximum, so that there is little distortion in the optical fibers 67. However, as shown in FIG. 8C, in the case where the optical fiber tape 63a is bent within the tape plane where the optical fibers 67 are arrayed, the length from the center line of bending 72 of one optical fiber becomes larger when the optical fiber comes near the end of the tape, so that the length takes a value near W/2 and a large distortion is produced in the optical fibers 67.
As shown in FIG. 7A, in the optical fiber cable where the plane portions of the optical fiber tape units 63a are put on the groove bottom portion 66 one on one successively, the optical fiber tape units 63a are bent by the spiral groove 62 in the case of an S-twisted or Z-twisted optical fiber cable having a not-inverted direction of twist. However, since the bending is as shown in FIG. 8B, little distortion is produced in the optical fibers 67.
However, in the case of an SZ-twisted optical fiber cable, a optical fiber tape unit is bent at the points F1 and F2 in the vicinity of the inverted portion of the spiral groove 42 shown in FIG. 5B, in the view from the outside of the groove as shown in FIG. 8C. That is, the optical fiber tape unit 63a is bent within the plane where the optical fibers 67 are arrayed so as to receive bending having a curvature center on the left of the left wall 64 or on the right of the right wall 65. As a result, there arises an excessive distortion in the leftmost and rightmost optical fibers 67 in the optical fiber tape unit 63a so as to lose its reliability. Therefore, in the SZ-twisted optical fiber cable, there arises a problem on the reliability of the optical fibers 67 in the case where the optical fiber tape unit 63a or the stack of optical fiber tape units 63 is accommodated in the groove.